Even as a girl, who has her own pair, and who likes them on others, I can't tell you exactly what their appeal is. I feel somewhat vulnerable when mine are massaged or licked, and I think that's part of the intimacy. Additionally, there are sensitive nerve endings in the nipples and areolae that heighten arousal, so stimulating that can be very beneficial for the ladies.

One could also ask what the fascination with people's backsides are. I'm not exactly an "ass girl" but I do like to grab it (my boyfriend's) now and then. But it's kind of the same question.

Also, I don't exactly believe the Freudian psychology about the connection between breasts as a sexually arousing body part and breast feeding. If that were the case, heterosexual women and gay men might like breasts, too. *shrug.*